Role of carboxylate bridges in modulating nonheme diiron(II)/O2 reactivity

A series of diiron(II) complexes of the dinucleating ligand HPTP (N,N,N,N-tetrakis(2-pyridylmethyl)-2-hydroxy-1,3-diaminopropane) with one or two supporting carboxylate bridges has been synthesized and characterized. The crystal structure of one member of each subset has been obtained to reveal for subset A a (mu-alkoxo)(mu-carboxylato)diiron(II) center with one five- and one six-coordinate metal ion and for subset B a coordinatively saturated (mu-alkoxo)bis(mu-carboxylato)diiron(II) center. These complexes react with O-2 in second-order processes to form adducts characterized as (mu-1,2-peroxo)diiron(III) complexes. Stopped-flow kinetic studies show that the oxygenation step is sensitive to the availability of an O-2 binding site on the diiron(II) center, as subset B reacts more slowly by an order of magnitude. The lifetimes of the O-2 adducts are also distinct and can be modulated by the addition of oxygen donor ligands. The O-2 adduct of a monocarboxylate complex decays by a fast second-order process that must be monitored by stopped-flow methods, but becomes stabilized in CH2Cl2/DMSO (9:1 v/v) and decomposes by a much slower first-order process. The O-2 adduct of a dicarboxylate complex is even more stable in pure CH2Cl2 and decays by a first-order process. These differences in adduct stability are reflected in the observation that only the O-2 adducts of monocarboxylate complexes can oxidize substrates, and only those substrates that can bind to the diiron center. Thus, the much greater stability of the O-2 adducts of dicarboxylate complexes can be rationalized by the formation of a (mu-alkoxo)(mu-1,2-peroxo)diiron(III) complex wherein the carboxylate bridges in the diiron(II) complex become terminal ligands in the O-2 adduct, occupy the remaining coordination sites on the diiron center, and prevent binding of potential substrates. Implications for the oxidation mechanisms of nonheme diiron enzymes are discussed.